1. Field of the Invention
This invention relates to an improved method for the production of articles of an aluminum or aluminum alloys having a colored anodic oxide film, and more particularly to a method for the production of colored aluminum or aluminum alloy which comprises subjecting an aluminum article having an anodic oxide film formed in advance thereon by an ordinary method to a treatment for modifying the pores in the anodic oxide film and subsequently to an electrolytic coloring treatment thereby effecting the coloration of the aluminum by means of optical interference.
2. Description of the Prior Art
In recent years, colored aluminum articles are extensively used in vehicles, building materials, exterior articles, etc. The colored aluminum articles adopted for these applications have been required to withstand prolonged exposure tp sunlight and weathering conditions without decolorization or discoloration. A process capable of producing colored aluminum articles having excellent weathereability and color fastness involves the electrodeposition of metal salts in the anodic pores of an aluminum article that has previously been anodised by a conventional method. The electrodeposition is carried out using the previously anodised articles or a number of them connected together electrically as one of the electrodes in an electrolytic bath containing one or more soluble salts of a metal such as nickel, tin or cobalt. Either AC or DC current is applied between this electrode and a suitable counter-electrode.
Previous electrolytic coloring provides color tones which are basically determined by the particular kinds of metal salts contained in the electrolytic bath. Only by varying the electrolytic conditions, these color tones may be changed in depth and shade. In commercial scale operations of previously known methods, the colors obtained were limited to dark shades including gray, bronze and black. This method has been incapable of producing colored aluminum articles in bright colors.
Moreover the previous methods have suffered from the disadvantage that whenever colored aluminum articles of a different hue are desired, there is the need to change the electrode to one containing different metal salts or to include baths of different compositions in the color anodising facility.
Recently, as a measure to remove this disadvantage of the conventional electrolytic coloring method, a method has been developed which, as disclosed in U.S. Pat. No. 4,066,816, Japanese Patent Publication No. 13860/1979 and Japanese Patent Publication No. 23658/1979, enables aluminum articles to a very wide range of bright color tones to be freely obtained in the same electrolytic bath by simply adjusting the duration of electrolytic deposition step.
This method primarily comprises subjecting a previously anodised aluminum article having an anodic oxide film formed thereon in advance to a treatment for the modifying of the anodic oxide film prior to the electrolytic coloring treatment by a metal salt thereby enlarging the volume of at least the bottom of the pores in the film. In the subsequent electrolytic treatment of this modified anodic film the upper surface of the deposits in the pores are at substantially the same distance from the barrier layer which separates the oxide from the aluminum substance and the process parameters are chosen so as to make this distance of the order of the wavelength of visible light. The upper surface of the deposits in the pores and the barrier layer are capable of reflecting light and the anodic film treated in this way is colored as a result of optical interference. By adjusting the parameters of the electrolytic treatment this process can be used to produce anodic films which are colored in bright color tones varying in the sequence of purple, indigo, blue, green, yellow, orange and red depending on the change of the duration of the electrolytic treatment, namely on the change in the thickness of the layer of the electrolytic deposits. The color tones obtainable by this method, therefore, have much more variety than those obtained by the conventional coloring method. Moreover, this method has a great economic advantage that aluminum articles of a wide variety of color tones are produced at will in the electrolytic bath containing one and the same metal salt in a single electrolytic bath of a chosen composition. In this method, however, when applied to aluminum articles of complicated shape difficulties may arise in achieving color uniformity.
To overcome the non-uniformity of color tones imparted to colored aluminum articles by the electrolytic coloring method utilizing interference colors, Japanese Patent Publication No. 128547/1978, for example, discloses a method which comprises forming a barrier-type oxide film as an intermediate treatment between a treatment for the modification of the anodic oxide film in preparation for coloring by optical interference and the electrolytic coloring treatment and thereafter performing an AC electrolytic coloring in an electrolytic bath containing one or more salts.
This method subjects an aluminum article to the barrier-type oxide film modification as an intermediate treatment for the purpose of reinforcing the barrier layer throughout the entire anodic oxide film and, at the same time, allowing a barrier film of an increased thickness to be preferentially formed in the portion of the aluminum article which is more susceptible to the flow of electric current, namely the portion in which coloring would occur more quickly and change in color tone would occur more readily during the electrolytic coloring step. The adjustment of the thickness of the barrier layer therefore improves color uniformity and for a given voltage, electrolyte and electrode configuration, and, it also lowers the rate of coloring thus facilitating color control. However while the additional control that can be achieved by the step of thickening and evening up the thickness of the barrier layer is useful, problems remain when AC current is used in the electrolytic coloring process and some of these can be minimized by using DC current instead.